My backups are done to disk images stored on a Time Capsule. When SuperDuper opens the backup script, it mounts the Time Capsule automatically to locate the image, and then mounts the image as part of running the backup script. This works 99% of the time.

A couple weeks ago I noticed some strange problems on one of my backups:

The backup image only had about half of the files it should - it just stopped halfway through the disk.

My root drive was missing about 50GB of space.

Running SuperDuper would faithfully examine all files, but the backup image remained the same.

What turned out to have happened is the first half of the files went to the backup image (we'll call the volume "BackupImage"). The remaining files were copied to the local hard drive at "/Volumes/BackupImage/". It's as if the backup image was unmounted halfway through and SuperDuper kept copying to the mount point. Like I said - strange. Finding that was tricky, as DaisyDisk, WhatSize, and others ignore /Volumes for obvious reasons, resulting in the Finder reporting one size and third party utilities reporting another. I finally found the files, cleaned up, and moved on - subsequent backups were fine.

Last night, one of my backups did this again, only this time all of the files went to the local drive, and I only had 10GB free to begin with. Did you know Mac OS X handles zero space free impressively well? Nothing crashed, just logs failed to be written and SD had an error on screen.

For last night's backup, I had just moved the backup image to a different location on the Time Capsule and updated the copy script to reflect that, but I didn't touch the scheduling thinking it would just pick up the changes by running the updated copy script. Perhaps this wasn't wise; I'm not sure.

Any ideas on this one? I don't know if there's any way to avoid this, especially if the volume disappears out from under SD, as it appeared to the first time.